The present invention relates to an exhaust gas purification device with which combustion gases originating from combustion devices, in particular diesel engines, can be purified. With the exhaust gas purification device, at least one regeneration of the particulate filters and/or of the NOx catalyst can be achieved. A soot particulate filter and/or a catalyst for reducing the content of nitrous gases (NOx catalyst) is thereby introduced into the exhaust gas flow of the combustion device. The central thought of the present invention is based on the fact that, in a preceding pyrolytic process, a gas is produced from liquid or gaseous carbon-comprising combustibles by means of a pyrolysis process with the exclusion of oxygen or oxygen-comprising gases or water vapour, said gas predominantly comprising hydrogen or a synthesis gas, i.e. carbon monoxide and hydrogen. This pyrolytically produced hydrogen-rich gas mixture/synthesis gas is used for the purpose of heating a particulate filter via an oxidation catalyst or LNT (Lean Temperature NOx Trap). This pyrolytically produced hydrogen-rich gas mixture/synthesis gas is used in addition for reducing the nitrous gases in the NOx storage catalyst. The present invention likewise relates to a corresponding method for the gas purification, suitable catalysts for the pyrolysis process and also a corresponding pyrolysis reactor.
The proportion of new registrations of diesel automotive vehicles in Germany has more than doubled from 1993 to 2003 from 19% to 42%. As a reason for this, there should be mentioned above all favourable fuel consumption and good driving properties based on a high torque at low speeds. At the same time, the exhaust gas limits have been made significantly stricter in Europe in recent years, in particular the particulates (PM) and the NOx emissions are intended to be further reduced. FIG. 1 (Department for the Environment and Transport, Baden-Württemberg, 2005) displays the permissible emission limits for commercial vehicles in Europe.
With the introduction of the Euro 4 standard in 2005, the upper limit for particulate emissions has been greatly reduced relative to 1993.
FIG. 2 shows how the particulate- and NOx emissions to date could be reduced by engine measures. In addition, the reduction of particulate and NOx emission limits is represented by the European exhaust gas standards. With the most modern engine technologies, the Euro 4 standard can be complied with. In view of the Euro 5 standard (introduction 2009), the use of particulate filters and measures for NOx reduction becomes absolutely necessary.
In addition the emission limits in force are also introduced for tractors and off-road vehicles. An extension to stationary applications will likewise follow.
Subsequently, measures known from the state of the art for regeneration of particulate filters or NOx removal, as have been used with exhaust gas purification processes to date, are described:
1. Regeneration of Articulate Filters
Particulate filters are loaded with particulates until a maximum permissible exhaust gas counter-pressure is reached. Then they must be regenerated. For the regeneration, normally exhaust gas temperatures of more than 600° C. at the filter entrance are required. Such high temperatures are however normally achieved only at high speeds of rotation at operating points near to full load. For this reason, aids are required for the particulate combustion both in engines in mobile use and for stationary uses in block-type thermal power plants.
Basically, a differentiation can be made between active and passive regeneration. However, in practice frequently combinations of both are used. FIG. 3 shows a list of current methods for regeneration of particulate filters.
In the case of active regeneration processes, the exhaust gas or the particulate filter is heated by separate energy to the required oxidation temperature.
The systems of passive regeneration enable filter regeneration under specific operating conditions without specific initiation of the oxidation process. No separate energy is required in passive regeneration.
A current method for regeneration of the particulate filter is the combination of CRT (Continuously Regenerating Trap) and post-injection. The CRT system is a continuously operating regeneration system. An oxidation catalyst which oxidises the NO in the exhaust gas to form NO2 is hereby fitted in front of the particulate filter. The NO2 can then oxidise the particulates at significantly lower temperatures than molecular oxygen in the exhaust gas (approx. 450° C.). According to the operating state, the necessary NO2 quantity cannot however be provided. If the exhaust gas counter-pressure exceeds a specific value because of the particulate loading, fuel is injected in front of the oxidation catalyst. The fuel oxidises in the catalyst and consequently heats the particulate filter. The particulates can then be combusted with the NO2 and the oxygen.
Particulate filters have not yet been able to date to be effectively regenerated at all operating points of an automotive vehicle. In particular during low load operation (for example in town operation), the exhaust gas temperature is frequently too low to oxidise the injected fuel completely. In the case of low exhaust gas temperatures the result can be condensation effects on the oxidation catalyst, the injected fuel can then coat the catalyst and possibly damage it.
2. NOx Removal
At present there are two systems for removing NOx, the SCR method and the NOx storage catalyst (LNT, Lean NOx Trap).
In the case of the SCR method, the reduction agent (a urea-water mixture) is conducted into the exhaust gas line of the engine. In the hot exhaust gas, the urea is converted to form ammonia. In the SCR catalyst, the nitrogen oxides are converted with ammonia to form water and nitrogen. This method is used at present with lorries. The SCR method is very efficient at temperatures >300° C., at exhaust gas temperatures <200° C. rather ineffective.
During “lean” operation (engine air ratio >1), the nitrogen oxides in the exhaust gas are stored temporarily in an NOx storage catalyst. The nitrogen oxides are thereby oxidised in the catalyst layer to form NO2 and subsequently adsorbed on the storage material. In the intermittent operation, the storage catalyst is loaded and regenerated again. The regeneration is effected in the substoichiometric engine operation (engine air ratio ≦1). The incorporated NO2 is thereby converted with the reduction agents HC (hydrocarbons) and CO to form N2. The storage material is regenerated and is available for new NOx incorporations. This system is of interest above all for automotive vehicles since, in comparison with the SCR method, it can be constructed more compactly. The regeneration is normally effected at temperatures of >250° C. In town for example, the storage catalyst can only be heated up to 150 to 180° C. because of the low waste gas temperature. A reduction of the storage catalyst is thereby possible only in a restricted manner.
There are at present various possibilities for producing a synthesis gas. The current reforming methods (such as e.g. steam reforming CPox (catalytic partial oxidation) and ATR (autothermal reforming) can be used. In the case of steam reforming, water vapour is required in addition to the fuel. The reforming is an endothermic process, i.e. heat must be supplied from outside. The space velocity during steam reforming is low in comparison with ATR or CPox. In addition, carbon is formed at too low temperatures and/or too little water. In the case of partial oxidation, the combustible is converted with oxygen to form a synthesis gas. In the case of CPox, it is important that the process parameters, temperature and air ratio, are adjusted precisely since otherwise carbon is formed. Water vapour and air are required with ATR. Similarly to the above-described reforming methods, this method is susceptible to the formation of carbon.
The carbon blocks the active centres in the catalyst and is in addition the nucleus cell for formation of new carbon. The catalyst becomes inactive and must be regenerated. The catalysts used during the reforming are not designed for such regeneration cycles. The thereby occurring, higher temperatures can lead to sintering effects and significantly reduce the activity of the catalyst. The processes become inefficient. In addition, normally noble metals are used as active components. The catalysts which are used during the reforming are therefore very expensive. Systems for exhaust gas post-treatment must primarily be simple, cheap and robust.
Such a reforming unit for producing hydrogen by steam reforming, partial oxidation of hydrocarbons and/or mixed forms thereof is known from WO 2004/090296. The reforming reactor described here and also the synthesis gas produced therewith can be used for NOx removal and for regeneration of particulate filters. The reforming unit can be positioned directly in the exhaust gas space and be operated with exhaust gas. For the steam reforming, the water comprised in the exhaust gas and also the residual oxygen can be used.
Disadvantages of this method relative to pyrolysis are:                The catalyst which is used based on noble metals is very expensive.        The method is very susceptible to the formation of carbon.        Integration of the reformer in the exhaust gas space appears very difficult on the basis of the changing conditions and the susceptibility to the formation of carbon.        